In the practice of interfacial polymerization to produce polycarbonate, a mixture of bisphenol and a phenolic chain-terminator is phosgenated under interfacial reaction conditions in the presence of an organic solvent. The bisphenol is present as an alkali metal salt in an aqueous phase and the phosgene is dissolved in the organic phase. The chain-growing-reaction is strongly accelerated by a coupling catalyst, such as a tertiary amine. The chain-growing reaction is finished after all of the reactive end-groups, e.g., chloroformate end-groups, have reacted.
After the polymerization step is completed, the organic phase comprising the polycarbonate resin is separated from the aqueous phase. Typically, the amine catalyst is removed from the organic phase comprising the polycarbonate resin by extraction with an aqueous acid such as dilute hydrochloric acid; wherein the acidic wash water comprises the amine catalyst as the amine hydrochloride.
Failure to recover the amine catalyst from the dilute aqueous wash water is undesirable from both an environmental and economic perspective. Several multi-step methods to recover the amine catalyst have been reported. FIG. 1 illustrates, in the form of a flow chart, a conventional method of recovering amine catalysts in interfacial polycarbonate manufacturing processes. U.S. Pat. No. 5,759,406 discloses a method to recover salts of amine catalysts by adsorbing said compounds on an adsorbent resin thus freeing it from other ionic species then washing the adsorbed compounds free by means of a second aqueous solution. However, U.S. Pat. No. 5,759,406 is silent as to how to recover the free amine catalyst from the second aqueous wash. JP 2002-356549 and JP 2001-164033 disclose a two-step method which consists of (1) neutralizing the acidic wash comprising the salt of the amine catalyst then (2) employing an expensive and energy intensive steam distillation step to recover the amine catalyst from the aqueous solution. JP 2001-329059 discloses two methods to recover the amine catalyst. A first method is a two-step method consisting of (1) neutralizing the acidic wash comprising the salt of the amine catalyst and (2) extracting the resulting neutralized aqueous solution containing the amine catalyst with an organic solvent, then recycling the organic solution comprising the amine catalyst back into the interfacial polycarbonate manufacturing process. Alternatively, the second method is a three-step method wherein (1) the acidic wash is neutralized forming an aqueous solution comprising the amine catalyst, (2) extracting the amine catalyst from the neutralized aqueous solution, then (3) separating the organic solvent from the amine catalyst by distillation. However, both methods have demonstrated draw backs. In the two-step method, a known side reaction is the reaction of the amine catalyst with the organic solvent, such as a chlorinated hydrocarbon, to form quaternary ammonium chlorides. When recycled back into the polymerization process even trace amounts of quaternary ammonium chlorides cause thermal and hydrolytic instability causing, among other things, increased color and/or haze, a high content of free bisphenol and phenolic hydroxyl groups, molecular weight degradation and/or lowered viscosity in the polycarbonate resin. It is well known that such quaternary ammonium salts are difficult to completely remove from the polycarbonate solution, thus contaminating the finished polycarbonate and causing said disadvantages. While the three-step method avoids the potential for forming quaternary ammonium chlorides, it does require an additional, expensive and energy intensive distillation process.
Therefore, there is a continuing need for an improved process to recycle amine catalysts in interfacial polycarbonate manufacturing processes which provides for (1) a simpler, more effective and cost effective recovery system of reusable amine catalyst minimizing potential loss to the environment and the formation of quaternary ammonium salts and (2) a polycarbonate resin with improved stability and lower color and/or haze.